Variable impedance device



1947- E. c. CORK ET AL 2,425,084

VARIABLE IMPEDANCE DEVICE Original Filed Feb. 17, 1944 Patented Aug. 5, 1947 Iran STATES PATA VARIABLE 'IIVIPEDANCE DEVICE Edward Cecil Cjork,-'Ealing, London; England, and Alan Dower-*Blumlein, deceased, late of-Ealing, London,.-England, by Doreen Walker, executrix, Lescudjack, Penzance, Cornwall, England, assignors to Electric and Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Original:application-February 17, 1944, Serial No. 522,830; Divided-sand this application June 2, 1945, Serial,No..597,250. InGreat Britain May SectionLTPubIi'c Law 690, August 8, 1946. Patent expires May 5, 1961 Claims. (01. 178-44) Thepresent invention relates toelectric cir-- cuit arrangements for use at highfrequencies andto apparatus for use in sucharrangements.

It is sometimes desiredv tomeasure or compare impedances at highzfrequencies-and conventional bridge circuit arrangements vsuch vasare normally used at lower frequencies-are found to be unsuitable, more particularly due to-the'v fact that at high frequencies radiations-tends to" occur and also undesired couplings take place between. different portions of the circuit, thus giving rise to instability and errors inmeasure-l ment.-

These difliculties may beovercomeby suitable screening, but such-screening is-frequently found todecrease the eiilciency of the arrangement.

Itis therefore an object of the. present inven-' tion to provide electric circuit arrangements suitable for the measurement or comparison of impedances at high frequencies in. which undesirable effects due to radiation or couplings are avoided without loss of efficiency.

According to one feature of the present invention there is provided an electriccircuit arrangement .for. use at high frequencies comprising a coaxial feeder having aninternal conductor and,

a surrounding, screen, a further conductor branching. from said screen at a pointintermediI- ate its ends and extending .therefromsubstantially parallel to. said screen to apoint adjacent one end thereof so that the impedance at the operating frequency between said end of said screen and the end of said further conductor is high, a connection between said internal conductor and a point on said further conductor;a sheath "substantially enclosing saidfeeder and-'said-furtherconductor and connected to the other end'of said screen, a source of high frequencyoscillations and 'a detector of high frequency oscillati'onsconnecte'd respectively between said internal conductorand said screen and between'sai'd sheath and said-branchpoint or apoint on-said-scr'een intermediate said branch point and-"said other" end thereof at which the impedance :is relatively high, or vice versa,- the arrangement being such that if an impedance is connected between said end of said screen and saidsheath and a further impedance is connected betweensaid end of said conductor and said sheath an electrical bridge network is formed.

It is a further object of the present invention to provide a variable resistance'specially adapted for use in an arrangement according to the above mentioned feature.

According to a further feature of the present invention, there is provided a variable resistance for use at high frequencies comprising a coaxial;

line having ;a central conductor and a surrounding screen substantially one quarter of a wavelength long at the operating frequency and effectively terminated-at its end by resistances which are inverse about the characteristic impedance of said line, the arrangement being'such that the impedance at any point on the inner conductor of said line is always a substantially pure resistance, themagnitude of which is dependent upon the position of said point.

If desired, said variable resistance may form part of a variable impedance comprising a vari-- able reactance in the form ofa coaxialline having ,a'movable short-circuiting piston adapted'to control the value of said reactance.

In order that the invention may be more clearly understood and readily carried into effect, it will now be describedin greater detail by reference to the accompanying drawin of which Figure 1 is a schematic circuit diagram of one embodiment thereof; and Figure 2 is a schematic circuit diagram of a modification thereof. Simi- .lar reference numerals are applied to similar elements throughout the drawing.

Referringnow to Figure 1 of the drawing, it will be seensthat thearrangement comprises a coaxial feeder having an internal conductor l and a surrounding screen 2 which branches at points into portions 4 and 5 forming a U-shaped structure, the internal conductor I continuing within the portion 4 and emerging therefrom at the upper end thereof to'make contact with the upper end of portion 5 at the point 6. Alternatively, said internal conductor may emerge at ax'point intermediate the point 3 and the upper end of portion 4 and make contact with the portion 5 at a point intermediate points 3' and 6.

Ancylindrical sheath l surrounds the coaxial feeder referred to and is closed at its lower end by a conducting annulus 8 which is joined to the screen 2 as shown. The upper end of said sheath 1 divides into the two side tubes 9 and Ill forming together with conductors I I and I2 connected to the upper end of portions 4 and 5 of the screen 2 respectively coaxial feeders to the outer ends of which impedances I3, M which are to be compared or balanced are respectively connected as shown. A source of high frequency oscilla tions I5 is connected between the conductor I and the screen 2 outside the sheath I as shown and a high frequency detector it, such as a diode, is effectively connected between the point 3 and an adjacent point on the sheath I as shown. Although the detector I6 is shown inside the sheath '1 it may, if desired, be arranged externally of the sheath 7 and connected to the point 3 and the screen I over a coaxial feeder. Any conventional signal indicator [1 providing suitable sensitivity may be connected to the detector (-6.

The lengths of the portions 4, 5 are chosen so 7 that at the frequency of oscillations providedby the source IS a high impedance is presented between their upper ends. Thus, each of these portions 4, 5 may be arranged to be approximately a quarter of a wavelength long at the operating frequency. With this arrangement, high frequency Voltages of large amplitude and opposite phase are developed in operation at the upper ends of portions 5, 5, so that the portions 4, 5 operate as high impedance bridge ratio arms.

Further, the distance between the point 3 and the points at which the annulus 8 is connected to the screen 2 is so chosen that the screen 2, annulus 8 and the sheath 7 form a concentric line short circuited at one of its ends and presenting a high impedance at point 3, The distance between the point 3 and the point at which the annulus 8 is connected to the screen 2 may be for example a quarter wavelength. Thus, the sheath 7 and the annulus 8 do not introduce any inconveniently low impedance in shunt with the detector Hi. It will be appreciated that, if desired, the distance between the point 3 and the points at which the annulus 8 is connected to the screen 2 may be longer than a quarter of a wavelength at the operating frequency and the detector l6 may be connected to an intermediate point of suitably high impedance so that, as before, the introduction of the sheath 7 and the annulus 8 do not give rise to any undesirably low shunt impedance.

It will be appreciated that the arrangement described constitutes a high frequency bridge circuit arrangement which is suitable for the measurement of high frequency impedance. Thus, for example, if the impedance i3 is an unknown impedance, its value may be determined by adjust ing the impedance it until the detector I6 gives a minimum indication. As the apparatus is completely screened, no undesirable radiation takes place and errors in measurement which might otherwise be caused by such radiation are avoided.

The positions of the source of high frequency oscillations t5 and the detector of high frequency oscillations l5 may if desired be reversed.

Any form of adjustable impedance which does not radiate may be used as impedance i i in the arrangement of Figure 1 above referred to, but

' according to a further feature. of the invention an impedance is provided which comprises a conveniently adjustable Variable resistance.

Referring now to Figure 2, the impedance comprises a variable resistance element in the form of a terminated coaxial line and a variable reactance element also in the form of a terminated coaxialline.

The variable resistance consists of the coaxial line comprising the cylindrical conductor 2i having an internal conductor '22 supported centrally within it. The ends of conductor 22 are secured to conducting pistons 23, 24 which are slidable within the conductor 2|, but the conductor 22 is insulated from said piston .23, 24. The ends of said conductor 22 are connected to said pistons 23, )24 by means of resistances 25, 25 respectively which may either be small resistances of the grid leak type or may be lengths of coaxial cable of sufliciently high attenuation to avoid reflections and therefore acting as non-inductive resistances of value equal to their characteristic impedance.

The variable reactance comprises a further cylindrical conductor 2'! branching out of the wall of conductor 2| and has a central conductor 23 supported within it. A conducting piston 29 is arranged to slide on said conductor 28 within said conductor 29 so as to form a connection of low impedance between said conductors 21 and 28. The conductor 28 is slidably connected to the conductor '22.

A cylindrical conductor 38 and central conductor 3| are also preferably provided for effecting connection to the common point of conductors 22 and 28. Thus, the conductor 30 preferably branches out of the wall of conductor 2| at a point opposite to the point at which the conductor '21 branches therefrom so that conductors 2i and 38 are coaxial and conductor 21 is an extension of conductor 28. The variable impedance can then be conveniently connected to the arrangement shown in Figure 1 by connecting conduotors 30 and 31 to the conductors Hi and 32 of Figure 1 respectively.

It can be shown that if the length of conductor 22 is such that it forms with conductor 2| a coaxial line which is substantially a half wavelength long at the operating frequency and if the terminating resistances 25 and 26 are equal and have the value R different from the characteristic impedance Z of conductors 21 and 22 considered as a coaxial line, then the impedance at any point in the conductor 22 is always resistive and has a value equal to cos (H- sin 0 where 0 is the electrical length between one of the pistons 23, 24 and the point at which the conductor 28 is joined to the conductor 22. Thus, by sliding the conductor 22 and the pistons 23, 24 fixed thereto inside the conductor 2|, the resistive impedance at the point of connection between conductors '22 and 28 will vary from according to the positions of the pistons 23, 24. It will therefore be appreciated that in order to obtain a wide variation of resistance R should differ considerably from Z.

Also, by sliding the piston 29 within the tube 21, the reactive impedance at said point of connection can be varied. Thus, the reactance and the resistance between the conductors 39 and 31 can be varied independently at will.

If desired, the pistons 23, 24 may be fixed within the conductor 2! and the conductors 27, 30,

together with the conductors 28, 3! "may' be-arranged to slide along the conductors 2! and 22 so'that the commonvpoint of the conductors 28 and 3t successively-makes contact with different points on the conductor 22 and thereby causes the resistive component of the impedance across conductors 3E to be varied. It will also be appreciated that the-conductors '2'? and W need not be coaxial and need not'be arranged to be normal to the conductor 2i as shown. For example, the conductor 27 may becranked'so as-to have its axis parallel to the axis of conductor 22 if desired.

Further, although the conductor 22 has been stated to be preferably "a half wavelength long it may if desired be only a quarter of a wavelength long providing that the terminating resistances are arranged to be invers about the characteristic impedance of the conductors 2i and 22 considered as a coaxial line. Thus, if the terminating resistance at one end of the conductor 22 has a value R, the terminating resistance at the other end must have a value It will therefore be appreciated that the variable resistance is fundamentally constituted by a coaxial line a quarter of a wavelength long and terminated by means of resistances R and which resistances may if desired be constituted by further lengths of coaxial line suitably terminated. The half wavelength case which has been described with reference to the accompanying drawing may thus be regarded as a quarter of a wavelength line terminated at one end by a resistance R, and terminated at its other end by a resistance in the form of a further quarter wavelength of line in turn terminated by a resistance R.

It will be understood that if the terminating resistances such as 25 and, 25 have some small reactance component, this reactance component can be taken up by suitably adjusting the length of conductor 22 so that the terminations effectively become pure resistances.

The above described variable impedance may be used as the adjustable impedance M in the arrangement previously described with reference to Figure 1. The lengths of the conductors 3B and 3! may then conveniently be chosen so that the impedance between the upper end of the portion 4 in the Figure 1 referred to and the impedance i3 is equal to the impedance between the upper end of the portion 5 insaid Figure 1 and the common point of conductors 22 and 23 in the accompanying drawing, so that when the bridge is balanced the impedance l3 will be equal to the impedance to which said variable impedance is adjusted.

Apart from its use as a high frequency bridge for the measurement of impedance at high frequencies, the invention may also have other applications. For example, an aerial may be adjusted to match a feeder of some predetermined impedance by coupling one end of the feeder to the aerial and the other end of the feeder to the conductor and screenB in Fig. 1 previously referred to in place of the impedance I3. The impedance i i required-to balance the bridge may thenbe noted and the aerial adjustment modified until the bridge balances with a purely resistive impedance M, thus showing the absence of reflection from the aerial.

The arrangement may also be employed to neutralize coupling between high frequency circuits.

For example, in the detection ofaircraft, short pulses of high frequency oscillations may be radiated at intervals and the reflections of such pulses from aircraft received upon the same aerial as that used for the transmission of the pulses.

'As the pulses have to be radiated at high'power and as the receiver has to be very sensitive, difficulty may arise due to the coupling between transmitter and receiver via the aerial system causing high voltages to be sent up in the receiver during the transmission of the pulses. This diificulty can largely be overcome by utilizing the arrangement of Figure 1. Thus, the generator l5 may be the transmitter, the detector It the receiver, the impedance E3 the aerial and the impedance M an impedance equal to that of the aerial. As the arrangement forms a balanced bridge substantially no voltage from the transmitter l5 will be set up across the receiver it. So far as high frequency oscillations received by the aerial it are concerned, however, the bridge is not balanced since similar oscillations are not developed in impedance 54, so that the oscillations received in the aerial l3 will be fed to the receiver it.

We claim as our invention:

1. A variable resistance for use at high frequencies comprising a coaxial line having a central conductor and a surrounding screen sub stantially one quarter of a wavelength long at the operating frequency and effectively terminated at its ends by resistances which are inverse about the characteristic impedance of said line, the arrangement being such that the impedance at any point on the inner conductor of said line is always a substantially pure resistance, the mag nitude of which is dependent upon the position of said point.

2. A variable resistance according to claim 1 in which one of said terminations is a further coaxial line substantially identical with said first mentioned line and terminated by a resistance equal to that terminating the other end of said first mentioned line so that the arrangement forms a coaxial line substantially half a wavelength long at the operating frequency and terminated at each end by a resistance differing from the characteristic impedance of said line.

3. A variable resistance according to claim 1 in which each end of said central conductor is fixed to and insulated from a conducting piston sliding within said screen and in which said screen is provided with an aperture from which a further screen branches, said further screen having a further central conductor sup-ported centrally within it so as to form a coaxial feeder therewith and slidably connected to said first, mentioned central conductor, the arrangement being such that the resistance effectively placed across one end of the coaxial feeder formed by said further screen and said further conductor can be varied by sliding said pistons and said central conductor fixed thereto.

4. A variable resistance according to claim 1 in which each end of said central conductor is fixed to and insulated from a conducting piston fixed within said screen and in which said screen is provided with an aperture from which a further screen branches, said further screen having a further central conductor supported centrally within it so as to form a coaxial feeder therewith and slidahly connected to said first mentioned central conductor, the arrangement being such that the resistance efiectively placed across one end of the coaxial feeder formed by said further screen and said further conductor can be varied by sliding said pistons and said central conductor fixed thereto.

5. A variable impedance comprising a variable resistance according to claim 1 and a variable reactance in the form of a coaxial line having 15 1,935,313

a movable short circuiting piston adapted to control the value of said reactance.

EDWARD CECIL CORK, DOREEN WALKER,

Formerly Doreen Blumlein, Executrz'a: of the Estate of Alan Dower Blumlein, Deceased.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Feldman Nov. 14, 1933 Nurnber 

